1. Field of the Invention
The present invention relates to a photoelectric converting film stack type solid-state image pickup device, which is configured by stacking photoelectric converting films on a semiconductor substrate in which a signal readout circuit is formed in the surface.
2. Description of the Invention
A prototype of a photoelectric converting film stack type solid-state image pickup device is disclosed in, for example, JP-A-58-103165. The solid-state image pickup device has a configuration in which three photosensitive layers are stacked on a semiconductor substrate, and electric signals for red (R), green (G), and blue (B) respectively detected by the photosensitive layers are read out by MOS circuits formed in the surface of the semiconductor substrate.
Solid-state image pickup devices having such a configuration have been proposed in the past. Thereafter, the techniques of developing a CCD image sensor and a CMOS image sensor in which many light receiving portions (photodiodes) are arranged in a surface portion of a semiconductor substrate and color filters of red (R), green (G), and blue (B) are stacked on the light receiving portions have been significantly advanced. At present, an image sensor with several million light receiving portions (pixels) integrated on the surface is mounted on a digital still camera.
The techniques of developing a CCD image sensor and a CMOS image sensor are advanced close to their limits or to the level where each light receiving pixel has an opening of about 2 μm that is on the order of the wavelength of incident light. Therefore, the techniques face a problem in that the production yield of photo-charges is poor.
The upper limit of the amount of photo-charges which can be accumulated in one miniaturized light receiving pixel is low or about 3,000 electrons. It is difficult to clearly express 256 gradations by using such a small amount of photo-charges. Therefore, it is hardly expected that an image sensor which is superior to the related-art one in image quality and in sensitivity is realized by a CCD or CMOS device.
The solid-state image pickup device proposed in JP-A-58-103165 has attracted attention as a device which can solve these problems, and image sensors are then proposed in Japanese Patent No. 3,405,099 and JP-A-2002-83946.
In the image sensor disclosed in Japanese Patent No. 3,405,099, a photoelectric converting layer is formed by dispersing ultrafine particles of silicon in a medium, three such photoelectric converting layers in which ultrafine particles have different diameters are stacked on a semiconductor substrate, and the photoelectric converting layers generate electric signals corresponding to the amounts of received light of red, green, and blue, respectively.
The image sensor disclosed in JP-A-2002-83946 is configured in a similar manner, or so that three nanosilicon layers of different particle diameters are stacked on a semiconductor substrate, and electric signals of red, green, and blue detected by the nanosilicon layers are read out to accumulation diodes formed in a surface portion of the semiconductor substrate.
In the image sensor disclosed in FIGS. 5 and 6(b) of JP-A-2003-332551, red-color filter photodiodes and blue-color filter photodiodes are disposed in a surface portion of a semiconductor substrate in the same manner as the related-art CCD or CMOS image sensor, and only one green-detection photoelectric converting film is disposed above the semiconductor substrate in place of a green-color filter photodiode.
FIG. 5 is a sectional diagram showing two pixels of the related-art photoelectric converting film stack type solid-state image pickup device. A CMOS signal readout circuit is formed in a surface portion of a semiconductor substrate 1 which serves as a ground of the photoelectric converting film stack type solid-state image pickup device. Referring to FIG. 5, a heavily-doped impurity region 2 for accumulating a red signal, a MOS circuit 3 for reading out the red signal, a heavily-doped impurity region 4 for accumulating a green signal, a MOS circuit 5 for reading out the green signal, a heavily-doped impurity region 6 for accumulating a blue signal, and a MOS circuit 7 for reading out the blue signal are formed in a surface portion of a P-well layer 1 formed in an n-type semiconductor substrate.
Each of the MOS circuits 3, 5, 7 is configured by impurity regions which are formed in the surface of the semiconductor substrate, and which are used for a source and a drain, and a gate electrode which is formed via a gate insulating film 8, respectively. An insulating film 9 is formed-above the gate insulating film 8 and the gate electrodes to flatten the surface, and a light shielding film 10 is stacked on the insulating film. In many cases, the light shielding film is formed by a metal thin film, and hence a further insulating film 11 is formed on the light shielding film.
Signal charges accumulated in the heavily-doped impurity regions 2, 4, 6 for accumulating respective color signals are read out to the outside by the MOS circuits 3, 5, 7.
Pixel electrode films 12 which are separated from one another so as to correspond to respective pixels are formed on the insulating film 11 shown in FIG. 5. For each of the pixels, the pixel electrode film 12 for the pixel is electrically connected through a columnar electrode 13 to the heavily-doped impurity region 2 for accumulating the red signal. The columnar electrode 13 is electrically insulated from the other components except the pixel electrode film 12 and the heavily-doped impurity region 2.
A red-detection photoelectric converting film 14 is formed above the pixel electrode films 12, and a transparent common electrode film 15 is formed above the photoelectric converting film. The photoelectric converting film 14 and the common electrode film 15 are not required to be disposed for each pixel, and are formed flatly over the entire face of the semiconductor substrate by respective single-film configurations.
Similarly, a transparent insulating film 16 is formed above the common electrode film 15, and transparent pixel electrode films 17 which are separated from one another so as to correspond to respective pixels are formed above the insulating film. For each of the pixels, the pixel electrode film 17 for the pixel is electrically connected by a columnar electrode 18 to the heavily-doped impurity region 4 for accumulating the green signal. The columnar electrode 18 is electrically insulated from the other components except the pixel electrode film 17 and the heavily-doped impurity region 4. A green-detection photoelectric converting film 19 is formed above the pixel electrode films 17 by a single-film configuration in the same manner as the photoelectric converting film 14. A transparent common electrode film 20 is formed above the photoelectric converting film.
A transparent insulating film 21 is formed above the common electrode film 20, and pixel electrode films 22 which are separated from one another so as to correspond to respective pixels are formed above the insulating film. For each of the pixels, the pixel electrode film 22 for the pixel is electrically connected by a columnar electrode 26 to the heavily-doped impurity region 6 for accumulating the blue signal. The columnar electrode 26 is electrically insulated from the other components except the pixel electrode film 22 and the heavily-doped impurity region 6. A blue-detection photoelectric converting film 23 is formed above the pixel electrode films 22. A transparent common electrode film 24 is formed above the photoelectric converting film. A transparent protective film 25 is formed in the uppermost layer.
When the light comes on the solid-state image pickup device, photo-charges corresponding to the amounts of the incident color lights of blue light, green light, and red light are excited in the photoelectric converting films 23, 19, 14, respectively. When a certain voltage is applied between the common electrode films 24, 20, 15 and the pixel electrode films 22, 17, 12, the photo-charges flow into the heavily-doped impurity regions 6, 4, 2, and are then read out as the blue, green, and red signals by the MOS circuits 7, 5, 3, respectively.
In the photoelectric converting film stack type solid-state image pickup device having the configuration shown in FIG. 5, the twelve films in total, or the pixel electrode film 12, the red-detection photoelectric converting film 14, the common electrode film 15, the insulating film 16, the pixel electrode film 17, the green-detection photoelectric converting film 19, the common electrode film 20, the insulating film 21, the pixel electrode film 22, the blue-detection photoelectric converting film 23, the common electrode film 24, and the protective film 25 must be stacked on the insulating film 11 on the surface of the semiconductor substrate.
In producing such a photoelectric converting film stack type solid-state image pickup device, in order to improve the production yield and to lower the production cost, it is necessary to reduce the number of production steps. When even one step of stacking the twelve films is omitted, the production yield is correspondingly improved, and the production cost is lowered.
In the case where such a photoelectric converting film stack type solid-state image pickup device is to be produced, the portion on the side of the semiconductor substrate can be formed in the same manner as that in the related-art CCD or CMOS image sensor, and the related-art technique of producing a semiconductor device can be employed without modification. Moreover, also the photoelectric converting films that are stacked on the semiconductor substrate, electrode films for sandwiching the photoelectric converting films, and insulating films can be easily formed using a film-forming method based on a printing technique, the spraying method, the vacuum evaporation method, the sputtering method, the CVD method, or the like.
However, conductor lines by which a signal readout circuit formed in the semiconductor substrate is connected to pixel electrode films for the photoelectric converting files stacked on the semiconductor substrate cannot be easily formed because such conductor lines must be formed as vertical ones which elongate in a direction perpendicular to the planes of the pixel electrode films.
For example, a solid-state image pickup device in which three or R-, G-, and B-photoelectric converting films are stacked has a configuration that the three color signals of red (R), green (G), and blue (B) are read out from one pixel. In this case, three vertical conductor lines must be disposed for each pixel, and the three vertical conductor lines have different heights.
Such vertical conductor lines can be formed by, after an insulating film is formed on the semiconductor substrate, repeating many times a work of etching away portions where the vertical conductor lines are to be formed, embedding a conductor in the portions, growing a photoelectric converting film, etching away portions where the vertical conductor lines are to be formed, and embedding the conductor in the portions.
However, the work has problems that alignment is not easily conducted, and that, when misalignment once occurs, the vertical conductor lines are not connected and photo-changer signals from the converting films cannot be transferred and read out. In order to form several million pixels in one solid-state image pickup device, it is required to form vertical conductor lines the number of which is three times that of the pixels. Moreover, a vertical conductor line through which photo-changer signals generated in an upper photoelectric converting film go to a signal readout circuit on a semiconductor substrate must be formed so as to be insulated from other photoelectric converting films and electrode films along the route, or not to make electrical contact therewith. This causes a problem that such a device is hard to produce and the production cost is high.